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Wettability is the ability of a liquid to spread out and leave a trace on a solid. This characteristic of liquids is related to the molecular interactions between the liquid molecules and the molecules of the surface.
The interaction between the molecules of the liquid and the surface, determines the contact angle θ, which is the angle formed between the exposed surface of the liquid and the contact surface with the solid.
When the contact angle is small (θ<90°) indicates that the wettability is high, and the fluid will spread over the surface; these surfaces are called hydrophilic. A large angle of contact (θ>90°) means a decrease in the fluid contact surface; in these case the wettability is low and the liquid forms a compact drop; these surfaces is called hydrophobic.
For a liquid such as water, a glass surface behaves as a hydrophilic surface, since the angle θ = 30°. So, it is said that water can wet a glass surface. A brass surface behaves as hydrophobic for water, since the contact angle is greater than 90°; in this case water can not wet the surface of brass.
This is because the glass is hydrophilic (water-liking). When we put a drop of water on a glass surface, it spreads and that's what makes it wet. We can quantify the degree of wetness in terms of the angle that water droplet makes with the glass surface. We can use a microscope for this purpose or a cellphone with a good enough camera. In either case, we will find that the angle is acute. We can also make the glass surface hydrophobic by treating it with silanes or some other coating and then the water droplet will be unable to wet the surface.
We can also explain the interaction in terms of forces. The adhesive forces between water molecules are less than the cohesive forces exerted by the glass and hence water spreads. With coating, the glass can be made more hydrophobic, so that these cohesive forces decrease in magnitude. For example, many of the high-end smartphones these days have a hydrophobic glass surface, i.e. water will form droplets on their surface that can be simply blown away by air, without any need to wipe the surface.
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